The present invention deals with thermosetting epoxy resin compositions. In particular, this invention deals with thermosetting epoxy resin compositions which give cured products with an extremely low internal stress and low coefficient of thermal expansion.
Cured epoxy resin compositions have been used widely as insulating materials in various types of electrical and electronic parts using methods such as transfer molding, injection molding, potting, casting, powder coating, dip coating and dripping because their electrical properties such as their dielectric character, volume resistivity and dielectric breakdown strength; their physical properties such as flexural strength, compression strength and impact strength, are excellent. However, epoxy resins are generally rigid resins and the mechanical stress imparted to the internal elements in electrical/electronic parts is quite significant. Thus, when electrical/electronic parts are encapsulated with an epoxy resin, the parts may not function normally or part of the element may be damaged. One cause of these phenomena seems to be the difference in the coefficients of thermal expansion between the element of an electrical/electronic part and the epoxy resin. The coefficients of thermal expansion of elements in electrical/electronic parts are very low while that of an epoxy resin is relatively high. Such large differences in the coefficient of thermal expansion result in the transmission of an excessive internal stress to the elements of electrical/electronic parts and other constituent materials after the part is encapsulated and cured. In addition, such differences in the coefficient of thermal expansion produce cracks in the epoxy resin itself and also cause a gap between the electrical/electronic part and the epoxy resin. The element deteriorates when water enters this gap.
Attempts have been made to overcome these disadvantages and at least two are known techniques. One is to add a resin to the epoxy resin which can impart flexibility. Although such a flexibility imparting agent prevents crack formation in the epoxy resin, it lowers the hardness, one of the most important characteristics of an epoxy resin. When heated, it also lowers the glass-transition temperature of the epoxy resin. This results in a loss of the high-temperature properties of the epoxy resin. The second technique is the addition of an inorganic filler with a lower coefficient of thermal expansion such as silica or alumina in an amount larger than usual. A cured product with the desired coefficient of thermal expansion can be obtained by this method. However, the flowability of the epoxy resin composition is substantially reduced which causes operations such as casting, transfer molding, potting, powder coating and dripping, to be essentially impossible. Since the Young's modulus of the epoxy resin is increased, the effect of lowering the internal stress by lowering the coefficient of thermal expansion may be reduced.
While considering the drawbacks of the prior-art technology, the present inventors carried out extensive investigations in an attempt to develop a thermosetting epoxy resin composition with a low coefficient of thermal expansion and low internal stress which does not cause blocking in powder coatings, and in which the physical properties of the cured product and the process flowability are not degraded and with which mold staining and bleeding onto the surface of the part of element does not occur during the molding process. As a result, the inventors found that the addition of a high-molecular weight, resin-linear organopolysiloxane block copolymer is very effective in lowering the coefficient of thermal expansion and lowering the stress, in thermosetting epoxy resin compositions.